FIG. 1 is an electrical schematic of a prior art electromotive traction system, as typically used in an off-highway vehicle. A DC output of an electrical power source 104 is connected to a DC bus 122 that supplies DC power to one or more traction motors 108. The DC bus 122 may also be referred to as a traction bus 122 because it carries the power used by the traction motor subsystems. A typical diesel-electric off-highway vehicle may include two traction motors 108 in a single axle implementation, one traction motor per each wheel, which in combination operate as an axle assembly, or axle-equivalent. It is noted that the vehicle may be also be configured to include a single traction motor per axle or may be configured to include four traction motors, one per each wheel of a two axle-equivalent four-wheel vehicle. In FIG. 1, each traction motor subsystem 124A and 124B comprises an inverter (e.g., inverter 106A and 106B) and a corresponding traction motor (e.g., traction motor 108A and 108B, respectively).
During braking, the power generated by the traction motors 108 is dissipated through a typical prior art electrical braking system 110. As illustrated in FIG. 1, electrical braking system 110 includes a plurality of contactors (e.g., contactors DB1 through DB5) for switching a plurality of power resistive elements electrically coupled between the positive and negative rails of the DC bus 122. Each vertical grouping of resistors may be referred to as a string. One or more power grid cooling blowers (e.g., blowers BL1, BL2) are normally used to remove heat generated in a string due to electrical braking.
FIG. 2 is a block diagram representative of one known physical layout of basic building blocks as implemented in a known traction system, and, more particularly, in a traction system that uses an electrical braking system that utilizes a combination of mechanical contactors and solid state power switches (e.g., semiconductor-based circuitry). As shown in FIG. 2, a cabinet 130 in the off-highway vehicle is configured to accommodate a group of four traction inverters with semiconductor-based circuitry (not shown) fully occupying four respective enclosures 1321, 1322, 1323 and 1324. Cabinet 130 in part accommodates a chopper circuit that uses semiconductor-based circuitry (not shown) arranged in a respective enclosure 1325 having the same form and fit as the inverter enclosures 1321, 1322, 1323 and 1324. It is noted that the semiconductor-based circuitry for the chopper circuit has a footprint that defines at least one cavity (e.g., cavity 133) in enclosure 1325. FIG. 2 further illustrates a contactor box 134 externally disposed with respect to cabinet 130 for accommodating one or more contactors (e.g., contactors DB1-DB5) that in combination with the semiconductor-based circuitry arranged in enclosure 1325 make up the chopper circuit for driving the electrical power-dissipating elements, such as resistive elements.
Electrical braking systems that use mechanical contactors tend to have a relatively high life cycle cost driven by maintenance of the contactors. Moreover, electrical braking systems that use mechanical contactors may not provide optimal performance as the drive system must wait for the contactors to close before retard power can be produced. This wait may be relatively long as it is subject to the constraints of a mechanical component. Space and weight also need to be efficiently allocated in connection with any retrofit installation that may be performed to the electrical braking subsystem of an off-highway vehicle.
Therefore, there is a need for providing a lower cost, lower maintenance and higher performance solution for controlling the flow of retarding (e.g., dynamic braking) power supplied into a resistor grid bank in an off-highway vehicle. It would be further desirable to provide a retrofit installation that improves the electrical braking system of an off-highway vehicle, without requiring extensive modifications.